Continuous use temperature of perfluoropolymers is the highest temperature at which the perfluoropolymer can be used for a long period of time while still retaining substantial strength. The length of time is 6 months and the retention of tensile property means that the loss in this property is 50% as compared to the property prior to exposure to the continuous use heating. This is the meaning of continuous use temperature herein. The tensile testing of the copolymer is done by removal of the copolymer test samples from the oven heated to the test temperature and then carrying out the tensile property measurement at ambient temperature (15-25° C.).
For tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, the continuous use temperature is 260° C., which is far less than the 302° C. to 310° C. melting temperature as disclosed on pp. 6 of S. Ebnesajjad, Fluoroplastics, Vol. 2, Melt Processible Fluoropolymers, The Definitive User's Guide and Databook, published by Plastics Design Library (2003). The copolymer melting temperature is for the highest melting copolymer, introduced in 1972, which is (tetrafluoroethylene/perfluoro(propyl vinyl ether). The melting temperature is the temperature corresponding to the DSC endothermic peak resulting from the phase change of the copolymer from the solid to the liquid state. The temperature that can be withstood by the copolymer is far less than its melting temperature, however, as indicated by the much lower continuous use (service) temperature.
The reduction in tensile property with prolonged heating indicates a deterioration of the integrity of the copolymer. The problem is how to improve the integrity of this copolymer so that it can be used at a temperature greater than its current continuous use temperature, i.e. the copolymer has a higher continuous use temperature.